1. Field of the Invention
The present invention relates to liquid crystal display devices for displaying an image using a liquid crystal and, more specifically, to a liquid crystal display device implementing a gray scale.
2. Description of the Background Art
Recently, liquid crystal display devices have been used as display panels for personal computers television receivers, portable telephones and the like. The liquid crystal display device has an advantage over a conventional display device because of its low power consumption and compactness.
FIG. 8 is a block diagram showing an overall structure of a conventional liquid crystal display device 500.
Referring to FIG. 8, liquid crystal display device 500 is provided with a liquid crystal display portion 1002 including a plurality of pixels 1001 arranged in a matrix. In a color liquid crystal display device, one pixel 1001 consists of pixels 1005 of R (Red), G (Green) and B (Blue). It is noted that, in the following description, the pixel corresponding to the above mentioned reference numeral 1005 is referred to as xe2x80x9cpixel.xe2x80x9d
Pixels 1005 are arranged in a matrix in liquid crystal display portion 1002. Liquid crystal display device 500 further includes vertical scanning lines 1010 and common lines 1012 arranged corresponding to rows of pixels and horizontal scanning lines 1011 arranged corresponding to columns of pixels.
FIG. 9 is a circuit diagram showing a structure of pixel 1005.
Referring to FIG. 9, pixel 1005 includes a liquid crystal display element 1102 having a pixel electrode and a common electrode arranged opposite to each other. Nodes respectively connected to the pixel electrode and common electrode of the liquid crystal display element are hereinafter referred to as a pixel electrode node Na and a common electrode node Nb.
The orientation of liquid crystal molecules in the liquid crystal display element changes in accordance with the potential difference between pixel electrode node Na and common electrode node Nb. The luminance of the liquid crystal display element thereby changes, enabling the luminance of each pixel to be controlled.
Pixel 1005 further includes: a storage capacitor 1103 arranged between pixel electrode node Na and common line 1012; and a TFT (Thin Film Transistor) element 1101 having its gate connected to vertical scanning line 1011 and electrically connected between signal line 1011 and pixel electrode node Na.
In pixel 1005, a positive voltage is applied to vertical scanning line 1010 (activation), so that TFT element 1101 is turned on and the potential level of horizontal scanning line 1011 is transmitted to pixel electrode node Na. Liquid crystal display element 1102 per se has a capacitance, and therefore liquid crystal display element 1102 and storage capacitor 1103 are charged with electricity by turning on TFT element 1101.
If vertical scanning line 1010 is inactivated and TFT element 1001 is turned off, on the other hand, the potential level of pixel electrode node Na is retained by storage capacitor 1103.
Returning to FIG. 8, liquid crystal display device 500 further includes a vertical scanning circuit 1003 for sequentially selecting the rows of pixels with a prescribed period, and a horizontal scanning circuit 1006 for supplying a display signal, which is a voltage signal corresponding to display data, to each column of pixels.
Vertical scanning circuit 1003 includes a shift register circuit 1004 and a buffer circuit 1005, and sequentially activates vertical scanning lines 1010 and applies a positive voltage thereto for sequentially selecting the rows of pixels with a prescribed period.
Horizontal scanning circuit 1006 includes a shift register circuit 1007 and a buffer circuit 1008 for sequentially selecting the columns of pixels with a prescribed period, as well as a switch 1009 arranged between display signal line 1013 and horizontal scanning line 1011. Switch 1009 is arranged corresponding to each row of pixels for connecting/disconnecting corresponding display signal line 1013 and horizontal scanning line 1011. Switches 1009 are sequentially turned on in accordance with a signal with a prescribed period. Display signal lines 1013 are respectively arranged for three pixels of R, G and B since liquid crystal display device 500 is a color display device.
Once display signals are written to pixels in one row of pixels by horizontal scanning circuit 1006, i.e., one row of pixels is scanned, vertical scanning circuit 1003 inactivates vertical scanning line 1010 that has been selected so far and applies a 0 or negative voltage thereto for selecting a next row of pixels, and activates the following vertical scanning line 1010 and applies a positive voltage thereto.
The next row of pixels is similarly scanned. When vertical scanning circuit 1003 scans all rows of pixels (this is also referred to as 1 frame), the leading vertical scanning line 1012 is again activated and a positive voltage is applied thereto. Thus, an image is displayed by sequentially writing display signals to all pixels in one frame.
As described above, in conventional liquid crystal display device 500, the luminance of each pixel corresponds to the potential level of pixel electrode node Na. Thus, an analog signal having a potential level corresponding to a gradation tone must be written from a horizontal scanning line to pixel electrode node Na to implement a gray scale of the pixels. Accordingly, digital/analog conversion is required if display data which is externally input to the liquid crystal display device is digital data.
A so-called area ratio gray scale method of dividing each pixel into a plurality of portions is known to provide a structure of implementing the gray scale in accordance with a display signal, which is a digital signal.
FIG. 10 is a diagram used for explaining the concept of the area ratio gray scale method.
Referring to FIG. 10, pixel 1005 is divided into a plurality of sub pixels SPX1 to SPXn (n: natural number) in the area ratio gray scale method. Each of sub pixels SPX1 to SPXn is independently turned on (maximum luminance) and off (minimum luminance) in accordance with a digital signal. Accordingly, if a sub pixel to be turned on is selected in accordance with the display signal of digital data in each pixel 1005, a gray scale is achieved in proportion to the area of the selected sub pixel.
However, the area ratio gray scale method suffers from a problem that the pixel must be divided into a larger number of sub pixels to increase the number of gradations. The increase in the number of sub pixels results in an increase in the size of each pixel whereby display resolution and quality are disadvantageously reduced.
An object of the present invention is to provide a liquid crystal display device capable of implementing a multiple-gray scale using digital data without impairing a display quality such as resolution.
Another object of the present invention is to provide a portable telephone and portable information assistance device provided with a liquid crystal display portion capable of implementing a multiple-gray scale by digital data without impairing display quality and continuously displaying the same image with low power consumption.
In short, the present invention is a liquid crystal display device including a plurality of vertical and horizontal scanning lines and a plurality of pixels.
The plurality of vertical and horizontal scanning lines are arranged in a matrix. The plurality of pixels are also arranged in a matrix. Each of the plurality of pixels includes M (M: natural number of at least 2) sub pixels. Each sub pixel corresponds to one of the plurality of vertical scanning lines and one of the plurality of horizontal scanning lines. Each sub pixel includes a liquid crystal display element as well as a common electrode and a pixel electrode arranged on either side of the liquid crystal display element. Each pixel further includes a sub pixel connection switch arranged between pixel electrodes of two of the M sub pixels. The sub pixel connection switch corresponds to one of the plurality of vertical scanning lines and one of the plurality of horizontal scanning lines and is turned on/off in accordance with the potential level of the corresponding vertical and horizontal scanning lines.
According to another aspect, the present invention is a portable telephone provided with a liquid crystal display portion. The liquid crystal display portion displays information in accordance with a digital signal.
The liquid crystal display portion includes: a plurality of vertical and horizontal scanning lines arranged in a matrix; a plurality of sub vertical scanning lines, a plurality of first reference potential lines, and a plurality of second reference potential lines, respectively arranged corresponding to the plurality of vertical scanning lines; and a plurality of pixels arranged in a matrix. Each pixel has M (M: natural number of at least 2) sub pixels. Each sub pixel corresponds to one of the plurality of vertical scanning lines and one of the plurality of horizontal scanning lines. Each sub pixel includes: a liquid crystal display element; a common electrode and a pixel electrode arranged on either side of the liquid crystal display element; a first switch element connecting the corresponding horizontal scanning line and a first internal node in accordance with the potential of the corresponding vertical scanning line; a capacitor for control for retaining the potential level of the first internal node; a connection switching circuit connecting one of the first and second reference potential lines to a second internal node in accordance with the potential level of the first internal node; and a second switch element connecting the second internal node to the pixel electrode in accordance with the potential of the corresponding sub vertical scanning line. Each pixel further includes a sub pixel connection switch arranged between the pixel electrodes of two of the M sub pixels. The sub pixel connection switch corresponds to one of the plurality of vertical scanning lines and one of the plurality of horizontal scanning lines and is turned on/off in accordance with the potential levels of the corresponding vertical and horizontal scanning lines.
According to another aspect, the present invention is a portable digital assistance device provided with a liquid crystal display portion. The liquid crystal display portion includes: a plurality of vertical and horizontal scanning lines arranged in a matrix; a plurality of sub vertical scanning lines, a plurality of first reference potential lines, and a plurality of second reference potential lines, respectively arranged corresponding to the plurality of vertical scanning lines; and a plurality of pixels arranged in a matrix. Each pixel has M (M: natural number of at least 2) sub pixels. Each sub pixel corresponds to one of the plurality of vertical scanning lines and one of the plurality of horizontal scanning lines. Each sub pixel includes: a liquid crystal display element; a common electrode and a pixel electrode arranged on either side of the liquid crystal display element; a first switch element connecting the corresponding horizontal scanning line and a first internal node in accordance with the potential of the corresponding vertical scanning line; a capacitor for control for retaining the potential level of the first internal node; a connection switching circuit connecting one of the first and second reference potential lines to a second internal node in accordance with the potential level of the first internal node; and a second switch element connecting the second internal node to the pixel electrode in accordance with the potential of the corresponding sub vertical scanning line. Each pixel further includes a sub pixel connection switch arranged between the pixel electrodes of two of the M sub pixels. The sub pixel connection switch corresponds to one of the plurality of vertical scanning lines and one of the plurality of horizontal scanning lines and the display portion of the corresponding vertical and horizontal portions display information in accordance with a digital signal.
Accordingly, a main advantage of the present invention is that the number of gradation tones can be increased by a switch connecting the pixel electrodes of the sub pixels when a gray scale is to be achieved in each pixel by turning on/off each sub pixel based on digital data.
Further, in the liquid crystal display portion, the number of gradations can be increased in accordance with a digital signal, and one of the first and second reference potential lines can be periodically connected to the pixel electrode of each sub pixel in accordance with the potential level retained at the internal node by the capacitor for control. Thus, when continuously displaying the same image, a writing operation at a high frequency for driving vertical and horizontal scanning lines needs not be performed. As a result, power consumption of the portable telephone and portable digital assistance device can be reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.